1. Field of the Invention
The present invention relates to a light adjustment device for a vehicle used to control illumination of the display panel, operating panel, or the like, of a vehicle.
2. Description of the Related Art
In a vehicle, illumination of various meters on the display panel and knobs on the operating panel, or the like, is performed by means of lamps, LEDs, and the like. A conventional light adjustment device for controlling illumination of these is constituted such that the amount of light emitted by the lamps or LEDs is adjusted by changing the output voltage of a control circuit by operation of a variable resistor.
A prior art example is described below with reference to FIG. 11 to FIG. 13. FIG. 11 is a circuit diagram of a conventional light adjusting device for a vehicle. Reference numeral 1 represents a terminal for connection to a power source, such as a battery, reference numeral 2 represents a lamp for illuminating an indicator 3, such as shown in FIG. 2, and reference numeral 4 represents an LED for illuminating meters 5, such as the speedometer and fuel gauge, and the like, as shown in FIG. 2. An anti-surge diode 6 and a current-limiting fixed resistance 7 are connected to the LED 4. Reference numeral 8 denotes a control circuit generating a constant voltage waveform and reference numeral 9 indicates a rotationally operated variable resistor which is connected to the control circuit 8. A current control transistor 10 is connected between the control circuit 8 and the lamp 2 and LED 4. Reference numeral 11 in FIG. 2 indicates an operating spindle 11 of the variable resistor 9; by rotating this operating spindle 11, the angle of the variable resistor 9 is changed. The variable resistor 9 exhibits resistance characteristics R10 as shown in FIG. 12. As can be seen, the resistance value is changed linearly in proportion to the operating angle of the variable resistor 9.
In this prior art light adjustment device for a vehicle, light adjustment is performed by rotating the variable resistor 9, whereby the output voltage from the control circuit 8 changes and the current flowing through the transistor 10 is altered accordingly, hence causing the intensity of illumination of the lamp 2 and LED 4 to change and therefore causing the brightness of illumination of the indicators 3 and the meters 5 on the display panel to vary.
FIG. 13 is a plot of illumination intensity (%) versus voltage showing the illumination characteristics of the lamp 2 and the LED 4 according to the prior art. As shown, the illumination characteristics L10 of the lamp 2 which illuminates the indicators 3 are expressed in a curve, whereas the illumination characteristics L20 of the LED 4 which illuminates the meters 5 are expressed in a straight line. Also, it can be seen that the LED 4 has a greater illumination intensity.
In the conventional light adjustment device for a vehicle, the light adjustment is performed by the variable resistor 9 simultaneously changing the respective intensities of illumination of the lamp 2 and LED 4 having such different characteristics. Since the variable resistor 9 has resistance value characteristics which are expressed in a straight line R10 as shown in FIG. 12, when the variable resistor 9 is rotated at a given angle whereby a given voltage is applied to both of the lamp 2 and the LED 4, the intensities of illumination of the lamp 2 and LED 4 at this given voltage will be different. As a result, a disparity is produced in the brightness of the indicators 3 which are illuminated by the lamp 2 and the meters 5 which are illuminated by the LED 4, and consequently the display will be difficult to view properly.